The present invention relates to a composite writing instrument having a common tubular casing accommodating two writing units such as two ball point pen units therewithin. In particular, the present invention relates to a composite ball point pen instrument in which two writing units are accommodated within a lower tubular casing and mutually spaced from each around the casing other by about 180.degree., and within an upper tubular casing is accommodated a cam mechanism rotatable around the axis of the casing through a certain angle and engageable with both of said two writing units for alternately shifting said writing units in the axial direction so as to advance said two writing units to a writing position through an opening in the tip of the lower tubular casing or to retract said writing units to a holding position located in said lower tubular casing by the rotational motion of said upper tubular casing around the axis.
There is already known a writing instrument provided with a lower tubular casing accommodating therewithin two ball point pen units with different color ink and an upper tubular casing accommodating therewithin a cam mechanism adapted to be engaged with said two ball point pen units. In such an instrument, the upper tubular casing is reciprocally rotatable around the axis of said lower tubular casing through a certain angle for alternately shifting said two ball point pen units to a writing position or a holding position. However, the cam mechanism of said writing instrument comprises, in general, a curved casing and a guide sleeve for holding one of the two ball point pen units in a writing position due to engagement of a slider fitted on said ball point pen unit with a notched portion formed on said curved casing.